In the field of radially expansible annular stents that are called upon, in use, to resist a radially inwardly directed force from surrounding bodily tissue, in order to maintain a bodily lumen patent, there is a contradictory design requirement. On the one hand, the stent must be strong enough to keep the lumen patent. On the other hand, the stent prosthesis must be flexible enough to accommodate movement of surrounding bodily tissue.
There are two archetypal stent forms. One of them has a stack of closed loop stenting rings, the length direction of the stent being along the length of the longitudinal axis of the annulus of the stent. The other archetype is the helical stent, in which the pattern of struts in the stent matrix performs a spiral path around the longitudinal axis, to create an annulus from one end of the stent to the other. Typically, each of the stenting loops is composed of closed periphery repeating unit cells. See EP-A-481365, FIG. 2. Typically, there are connector struts present, periodically through the annular matrix, to set the longitudinal spacing between adjacent stenting loops. See WO94/17754.
Such a stent is typically made from a seamless straight tubular workpiece so that its disposition, at rest, and relaxed, is that of a tubular cylindrical annulus. Typically, after implantation in the body, it is called upon to conform to an arcuate configuration of the bodily lumen in which it is placed. Such a change of shape necessitates the occurrence of strain within the matrix. That strain might not be homogeneously distributed throughout the matrix. Important for flexibility of the stent, after placement in the body, is a capacity for tolerating enough strain to give the stent, as such, enough flexibility to move with the body.
Another aspect of flexibility that is desirable when placing a stent is “radial conformability” by which is meant the ease with which succeeding turns of the stent can take or, after placement in tissue, different diameters clearly, when the struts connecting adjacent stenting rings have enhanced flexibility, an increase of radial conformability is in prospect.
Closed periphery unit cells of the stenting matrix are inherently rather well-adapted to provide the required resistance to the radially inwardly pressing force of the bodily tissue. In consequence, it is desirable for any connectors of unit cells, within the matrix, to deliver at least a substantial portion of the strain needed to allow the stent matrix to move with the body. Such flexibility in the connector links is not detrimental to the capability of the stenting loops to push the bodily tissue radially outwardly. For this reason, current stent designs often exhibit unit cells with simple straight strut peripheral portions, connected by connector struts that are not short and straight but long and thin. They are often meandering or arcuate or serpentine. There is discussed below, with reference to FIG. 1, showing an exemplary stent having connector struts of the serpentine kind This extra length provides the connectors with increased capacity to absorb strain and deliver flexibility to the stent, as such. However, building a stent annulus with convoluted or serpentine connectors adds to the complexity of manufacture and might not assist in meeting other government regulatory or quality control requirements.
It is an object of the present invention to ameliorate these difficulties.